Vehicles such as aircraft have a number of auxiliary systems which require the supply of compressed air such as environmental control systems which control cabin pressurization and air conditioning. In the prior art, the supply of compressed air for auxiliary systems in aircraft usually has been obtained from the main engine compressor wherein bleed ports are strategically located at one or more stages of the compressor. These bleed ports supply compressed air directly from the main engine compressor to the environmental control system and other auxiliary systems of the aircraft.
One disadvantage with the use of bleed ports is that the flow and pressure needs of the auxiliary systems often do not match the supply of compressed air available at the bleed ports located within the main engine compressor. This is due to variations in the pressure of the compressed air available as the power levels of the engine change during takeoff, cruising and landing, and/or because it is inconvenient or impossible to position a bleed, port at the appropriate compression stage within the main engine compressor to obtain a matched supply of compressed air for the auxiliary systems.
Even where bleed ports can be positioned at a compression stage within the main engine compressor which relatively closely matches the demand of the auxiliary systems, such matching is usually obtained only at certain altitudes. Often, there is a substantial waste of pressure work at normal cruising altitudes of the aircraft which becomes even worse as the aircraft flies lower. In addition, the pressure and flow rates of compressed air required by the auxiliary systems vary as the aircraft changes altitude and these variations often are not well matched to the power level and compression level operation of the aircraft engine. Generally speaking, the supply pressure of the compressed air from the main engine compressor to auxiliary systems of an aircraft is always higher than required. This leads to substantial inefficiency and lost pressure work.
One attempt in the prior art to solve some of the problems outlined above has been to provide an auxiliary compressor which is mechanically driven by the shaft of the compressor in the main engine. In these systems, a constant gear ratio arrangement has been employed in which the rotation of the main shaft of the engine compressor is directly transmitted to the auxiliary compressor to drive it at a proportional speed and produce compressed air for the auxiliary systems of the aircraft.
A problem with prior art systems employing an auxiliary compressor driven from the main engine at a constant gear ratio is that the mismatch problems between the supply and demand for compressed air are simply transferred from the main engine compressor to the auxiliary compressor. Design considerations dictate that the auxiliary compressor operating point and speed be set according to the worst operating point condition of the main compressor because of the constant gear ratio connection therebetween. Unfortunately, such settings are often far removed from the peak efficiency operating conditions of the auxiliary compressor. As a result, any advantages which could be theoretically obtained by using an auxiliary compressor to supply compressed air to the auxiliary systems instead of bleed ports in the main engine compressor are largely lost.